1. Field of the Invention
The invention relates to a process for the preparation of a chemical or petrochemical reaction catalyst, constituted by an oxide of heavy metals having on the surface the metal carbides and oxycarbides with a large specific surface. This catalyst is particularly suitable for the refining and transformation of petroleum products and in particular for reforming. The invention also relates to the catalyst obtained by this process.
2. Description of the Related Art
French patent application 89 04433 (Pechiney Electrometallurgie) describes a process for the production of carbides of heavy metals with a large specific surface for use as catalysts in the petrochemical industry. These carbides are obtained by reacting at a relatively high temperature on a carbon with a large specific surface, a volatile compound of the heavy metal, whose carbide it is wished to obtain. Thus, to a certain extent the carbide retains the memory of the specific surface of the starting carbide and retains a larger active surface than the prior art carbides, which makes it readily usable as a catalyst.
Subsequent research carried out by the same team of scientists has revealed the essential function played in the catalytic mechanism by the oxycarbides present on the surface and identified by X-ray photoelectron spectroscopy (XPS) analysis. French patent application 90 11633 (GIE Pechiney Recherche) thus describes an oxidation treatment performed at a temperature of approximately 300.degree. C. on metal carbides prepared according to the aforementioned application and which makes it possible to obtain said oxycarbides, which are responsible for most catalytic activity. This treatment must be followed by a reactivation period at 350.degree. C. in a hydrocarbon-hydrogen mixture, which is generally the mixture which is to be isomerized in the reforming operations.
U.S. Pat. No. 4,515,763 (Boudart) also describes a process for the production of carbides and nitrides of heavy metals, including molybdenum and vanadium intended for catalysis. This process is based on a principle opposite to that of French patent application 89 04433, because in this case a carburizing gas, generally methane, diluted in hydrogen is reacted on the solid metal oxide. The process also involves a precise temperature rise cycle between 768.degree. and 100.degree. K., a relatively high hydrocarbon concentration of at least 10% by volume, quenching once carburization is complete and preferably a very cautious passivation treatment in a weakly oxidizing gas, the product obtained being pyrophoric in contact with air, due to the fact that it is prepared in a highly reducing medium.